1. Field of the Invention
The present invention relates to a process for handling a green compact made by a press machine from a rare earth metal-based magnetic alloy powder such as an Fe—B—R based magnetic alloy powder, wherein R comprises at least one rare earth element. The present invention also relates to a rare earth metal-based magnet produced through such handling process.
2. Description of the Related Art
It is a conventional practice for producing a rare earth metal-based magnet to press a rare earth metal-based magnetic alloy powder into a predetermined shape in a magnetic field by a press machine, and to arrange green compacts produced in the above manner on a sintering support plate to transport them into a sintering furnace, where they are sintered.
In this case, a press machine 10 and a sintering support plate 15 shown in FIG. 12 are used for the handling of the green compacts made from the rare earth metal-based magnetic alloy powder by the press machine to transport the green compacts to the sintering support plate. The green compacts 1 formed into a predetermined shape from the rare earth metal-based magnetic alloy powder by the press machine are pushed out onto a stage 12 by a push-out means 11 such as pusher and subjected to a powder removing treatment in which a surplus magnetic powder around the green compacts 1 is blown away by a nitrogen gas or the like blown out of a powder removing device 13. Then, the green compacts are pushed out onto a transporting belt 14 by the push-out means 11. The green compacts are transported to near the sintering support plate 15 by the transporting belt 14 and then pushed out onto the sintering support plate 15 from the transporting belt 14 by a push-out means 16 such as a pusher. Thus, a large number of the green compacts can be arranged efficiently on the narrow sintering support plate of a simple construction and hence, the above-described steps are repeated, thereby allowing the succeeding green compacts 1 to sequentially push the already transported preceding green compacts to slide them on the sintering support plate 15, as shown in FIGS. 13 and 14. In this manner, all the green compacts 1 are arranged in a final transport position on the sintering support plate 15. In FIG. 12, reference character 10a designates an upper punch of the press machine 10; reference character 10b designates a die of the press machine 10; reference character 10c designates a box (feeder) for supplying the magnetic alloy powder to the press machine 10; and reference character 10d designates a magnetic field generating coil.
However, the rare earth metal-based alloy powder such as the Fe—B—R (R comprising at least one rare earth element) based magnetic alloy powder has a large hardness as compared with ferrite. For this reason, if such powder is pressed too strongly, the die is worn. If the powder is pressed at a high pressure, the orientation tends to be disordered, resulting in a degraded magnetic characteristic. Therefore, in order to provide a higher magnetic characteristic pressing force, the pressing pressure can be less risen and hence, the green compacts are liable to brittle and destroyed, as compared with ferrite. Particularly, a rare earth metal-based magnetic alloy powder made by the strip casting process and having an excellent magnetic characteristic has a small average particle size and moreover, has a narrow and sharp particle size distribution. Therefore, green compacts produced from such rare earth metal-based magnetic alloy powder are soft, a poor shaping property, and difficult to handle, as compared with a powder which is made by a mold-casting process and whose particle size distribution varied widely. A green compact made by pressing from a powder containing a lubricant such as an ester of an aliphatic acid added thereto is further brittle.
Because the green compacts are brittle as described above, it is necessary to handle the green compacts carefully by a transporting means such as a transporting belt, a pusher, a robot and the like. Especially, there is a problem that the powder removing treatment is time-consuming, and unless the powder removing treatment for the green compacts made in advance by pressing is finished, the pressing of the subsequent powder cannot be started, resulting in a significantly degraded efficiency of operation of the press machine.
To exhibit the magnetic characteristic sufficiently, it is necessary to conduct the pressing in a high magnetic field of 0.9 to 1.5 T and for this reason, it is necessary to demagnetize the green compacts by a counter magnetic field after the pressing. However, the perfect demagnetization cannot be achieved and for this reason, the powder scattered around the green compact is adsorbed. It is impossible to advance the process without carrying-out of this powder removing treatment and hence, an increase in efficiency of the powder removing treatment is a large subject.
The use of the sintering support plate having a high friction coefficient is preferred in order to ensure that the green compacts are prevented from slipping on the sintering support plate to come into close contact with another green compact, or to become fallen, during transportation of the sintering support plate to the sintering furnace. Particularly, the R—Fe—B based magnet is produced in a liquid-phase sintering manner. For this reason, if a very smooth support plate is used, neodymium (Nd) eluted during the sintering is deposited onto the support plate and hence, it is necessary to use a support plate having a high friction coefficient. For this reason, there is arisen a problem that the green compacts which are slid through a longer distance, i.e., arranged earlier, are cracked at their bottoms, and in a severe case, the green compacts are destroyed before the sintering. To push out the green compacts in a first row, the green compacts, if being pushed by a friction force corresponding to one green compact, can be slid on the support plate. However, it is necessary to push the green compacts in an n-th row by a friction force corresponding to an n-number of green compacts, and such friction force is applied locally to the green compacts in the n-th row. If such friction force is larger than the strength of the green compacts, the green compacts are crushed and destroyed. In addition, the green compacts in the first row are slid through a distance corresponding to the n-rows and for this reason, are chipped at their bottoms